Apparatuses for controlling a motor installed in an electric power steering system cause the motor to generate assist torque to be applied to a steering wheel via a steering mechanism, thus assisting the driver's steering operation of a steering wheel.
For example, Japanese Patent Application Publication No. 2008-137486, which will be referred to as a published patent document, discloses a control apparatus for an electric power steering system. The control apparatus determines a commanded current in accordance with an actual value of a current that flows through a motor and is measured by a current sensor. Then, the control apparatus supplies the commanded current to the motor to thereby adjust the assist torque to be applied to the steering wheel via a steering mechanism.
The control apparatus specially determines whether the steering wheel has reached an upper limit, i.e. a steering end, at which sudden stop of the motor may occur. The situation where the steering wheel reaches the upper limit will also be referred to as the steering limit situation.
Upon determining that the steering wheel reaches the upper limit, the control apparatus holds a value of the commanded current to the actual current value measured by the current sensor immediately before the occurrence of the steering limit situation. This limits the value of the commanded current to be lower than an actual value, i.e. an upper limit value, measured at the occurrence of the steering limit situation. This results in reduction of an impact applied to the steering mechanism due to the sudden stop of the motor.